Five species of Bartonella are emerging agents of infectious human disease. Because of their ability to parasitize erythrocytes and endothelial cells within the circulatory system, bartonelloses present with a wide array of cardiovascular manifestations including endocarditis, bacillary angiomatosis, peliosis, chronic bacteremia, and hemolytic anemia. Bartonella's parasitism of microvascular endothelial cells is characterized by a proliferative response triggered by a bacterial protein that we have termed Bartonella angiogenic protein (BAP). The resulting angiogenesis generates a pseudoneoplastic vascular lesion. In our work on BAP, we recently discovered an inhibitory protein from Bartonella henselae that significantly reduces the growth of human vascular endothelial cells, termed BIF. Production of BAP and BIF by Bartonella suggests that the pathogen plays an active role in regulating the growth of its host cell. However, neither the molecular nature nor mechanism of BAP or BIF are known. Therefore, the long-term goals of this project are to characterize BIF and its mechanism of action, using Bartonella henselae as a model for the genus. Specific goals to achieve these objectives will: 1) Characterize BIF at the molecular level using biochemical and molecular biological techniques, 2) Analyze BIF's mechanism by mapping domains necessary to its inhibitory activity by creating overlapping BAP deletion mutants and assaying their ability to inhibit vascular endothelial cell growth, 3) Examine BIF and BAP for antagonism in vitro and assess the angiostatic activity of BIF in vivo, and 4) initiate studies to elucidate BIF's mechanism by identifying and characterizing its cognate endothelial cell receptor(s). These experiments will generate valuable data on the molecular nature of a novel bacterial protein that can modulate the growth of human vascular endothelial cells and will help elucidate the mechanism whereby angiomatic and pseudoneoplastic disease manifestations of bartonellosis are generated during infection.